I've started reversing lenses for macro photography and I'm loving it, but something bothers me: a lot of articles say that it exposes the rear element of the lens. Are there protection rings available for the rear element when using a lens this way or do I need to do it myself like this one: http://www.instructables.com/id/Reversed-Lens-Filter-Adapter/? It seems nice but, even if I like duct tape, using it for something relative to light leaves me uncomfortable...

+1 - I have wondered this also when I use a reversal ring. Like the DIY method but finding this pre-manufactured would be great.
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LyndaMay 14 '12 at 2:05

Judging from the answers, comments and even the duplicate questions, If you ever consider the DYI route, you need to sacrifice a body cap, a lens rear cap and two step rings or two filters and you get a complete lens reversal kit. Glue a body cap to a step ring to thread the lens, and glue a ring to a rear lens cap to allow threading a filter or a regular lens cap on the rear of the reversed lens.
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JahazielNov 7 '14 at 20:09

The protection ring is attached to the rear of the lens (the lens is in reverse position via Retro Adapter Professional), where it holds a protective filter with 58mm thread (for UV filters, glass filters or polarizing filter) on.

Thus, the rear lens is protected against scratches and the electrical contacts are reliably protected against dust. If you do not have filters, you can also order UV filter.

Nice !! I was afraid this was only for canon but there ils also a model for nikon, sweet .but can you still manipulate the aperture with that on it ? If not, its kind of useless :/
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eephyneMay 16 '12 at 20:43

It protects your reversed lens. Changing the apperture when reversing a lens is only possible with a lens that has a manual apperture. I would use that kind of lens when doing reversed macro. ; )
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xtarsyMay 16 '12 at 22:24

I know that, but does the protection ring allow you to have access to the manual aperture ?
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eephyneMay 17 '12 at 8:00

Awww you mean aperture ring. Didnt think of it. Hum need to found an old lense now.
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eephyneMay 17 '12 at 8:19

When looking for reverse mounting a lens, I have had the same issue, where I wanted to try macro but did not want to leave the lens open to dust or damage. The option I came up with was to modify a rear lens cap.
What I did was to use a 50mm hole cutter to take out the center of the rear lens cap. I then trimmed down the remain wall to open out the hole size so I could screw into place a 52mm filter.
To be honest it was harder then I thought to "screw" the flter into place but I used some sandpaper wrapped around a tapered cup to smooth out the trimmed hole. The taper on the cup then allowed me to open up the hole size to clamp the filter into position. The thread on my filter was 52mm so i opened out the hole size to 51.5mm. This allowed me to force the filer into place without the risk ofr cracking the cap or filter.
This is a cheap option which can be mounted on any EF mount lens, I also use a 52mm lens cap on the cap so this also keeps finger mrks off the filter lens. Hope this helps you all out there.